Will front disc brakes off a 69 dart fit a 74 scamp???

Number 1
The 1969 bolt pattern is smaller, and it limits your choice of common wheel sizes, and you would have two different bolt patterns on your car.
Number 2
The K-H 4 piston calipers can be troublesome compared to the single piston calipers that are much more common.

I'm sure there are a couple more, but just those two should be enough. Get the parts off the car and advertise them here, get the front spindles and rotors and calipers from a 73-76 A body and bolt them on instead, and you may even be able to pocket some money.